In applications requiring that numerous items be weighed, it is important to reduce the measurement time for each item. To this end, weighing devices have been equipped with dampers which make it possible, at each weighing operation, to reduce the time needed for reaching equilibrium after a load has been placed on the weighing device.
The present invention is aimed at such a damper as well as at a weighing device provided with a damper. Different types of dampers are known for weighing devices such as, for example, those described in document EP 0309144 and in document U.S. Pat. No. 5,232,062.
A first type of damper for a weighing device is described with reference to FIG. 10 of EP 0309144 and is comprised of a container open upwards and filled with a liquid in which is immersed a plunger affixed to the measuring body. When the plunger oscillates, its displacements are damped by the passage of liquid through an annular constriction between the plunger and the container. Such dampers have the drawback that the container filled with a liquid is open and therefore, when transporting the weighing device the liquid must be removed or the container must be tightly closed. Owing to the fact that the container is open, dust or other contaminants can penetrate therein and modify the characteristics of the liquid, in particular its viscosity.
Furthermore, depending on the weight of the item being weighed, the rod of the plunger penetrates more or less deeply into the liquid, which modifies the upward pressure exerted upon the plunger and is hence detrimental to the accuracy of the measurement. The same holds true for the forces which are associated with the surface tension of the liquid and which cause the liquid to creep up along the walls of the container and along the rod of the plunger.
The same document EP 0309144 describes a second type of damper including two tight chambers filled with a liquid and connected via a constriction. When the weighing rod oscillates, the liquid passes alternately from one container into the other via the constriction, thus damping the oscillations of the measuring member. Such dampers are complex and bulky owing to the use of two sealed chambers. Furthermore, these chambers are completely filled with liquid and the accuracy of the measurement is influenced by changes in the barometric pressure.
Furthermore, the two types of dampers described above have a damping force which is constant, whatever may be the amplitude of the oscillations of the measuring member.
A third type of damper for a weighing device is known from document U.S. Pat. No. 5,232,062, which functions based on the dynamics of liquid films of a reduced thickness. The container containing the liquid is also open here, and the plunger which is immersed therein has a lower surface which is planar and parallel to the bottom of the reservoir. Furthermore, the annular clearance between the reservoir and the plunger is wide and does not restrict substantially the flow of liquid during the oscillations of the measurement device to which the plunger is affixed.
In this type of damper, the damping force is very high and is proportional to the square of the surface of the plunger and inversely proportional to the distance between the plunger and the bottom of the container.
In this type of damper, the force is so important that the movement is damped in one or two oscillations only. However, owing to the properties of thin films, a residual force remains acting on the plunger, which is detrimental to the accuracy of the measurement. Furthermore, such dampers can only be used in weighing devices wherein the amplitude of the oscillations of the measuring element is very small owing to the small distance of the plunger from the bottom of the container.
The object of the present invention is a damper for weighing devices which obviates the above-mentioned shortcomings of existing devices. This invention is furthermore aimed at providing a damper for a weighing device which can be readily transported, which is not bulky, wherein the damping force is not determined by the physical properties of thin liquid films, which does not produce any alteration in the accuracy of the measurement and wherein a component of the damping force is a function of the weight to be measured, in such a manner as to optimise the damping time, whatever may be the weight measured.
Accordingly, the object of the invention is a damper for weighing devices including a cavity in which a plate can move, which plate is designed for being connected to a weight measuring device, characterised in that the cavity is closed tightly by a flexible membrane; in that this cavity is partly filled with an oil, while leaving a volume of air or of a gas between the top of the oil and the membrane, the plate being totally immersed in the oil; in that the lower surface of the plate and the bottom of the cavity have shapes which are not planar and which are not parallel to each other, thus preventing the formation of a thin film of oil between them; in that the peripheral surface of the plate and of the peripheral wall of the cavity define therebetween a first constriction of which the transverse section is constant whatever the position of the plate in the cavity; and in that the peripheral area of the bottom of the cavity and the peripheral area of the lower surface of the plate facing the bottom of the cavity, define together a second constriction of which the transverse section varies depending on the position of the plate inside the cavity.
The lower surface of the plate can have a shape which is curved and concave, with a peripheral rim.
The bottom of the cavity can exhibit a planar annular area, a protruding annular median area and a central concave area. The upper face of the cavity 4 can be tightly closed by a cover exhibiting a central passage providing access to a connector member connecting the plate to the free end of the deformable rod and the periphery of the flexible membrane can be clamped between the cover and the upper edge of the cavity, the central area of this membrane being clamped between the plate and the connector member.
The invention is also concerned with a weighing device including a frame, a sensor provided as a deformable monolithic rod, of which one end is affixed to the frame while the other end receiving the load to be weighed is fastened to the plate of a hydraulic damper movable within a cavity, characterised in that this cavity is provided in the frame and in that it is tightly closed by a flexible membrane; that this cavity is partly filled with an oil leaving a volume of air or of a gas between the top of the oil and the membrane, the plate being totally immersed in the oil; in that the lower surface of the plate and the bottom of the cavity have shapes which are not planar and which are not parallel to each other, thus preventing the formation of a thin film of oil between them; in that the peripheral surface of the plate and of the peripheral wall of the cavity define therebetween a first constriction of which the transverse section is constant whatever the position of the plate in the cavity; and in that the peripheral area of the bottom of the cavity and the peripheral area of the lower surface of the plate facing the bottom of the cavity define together a second constriction of which the transverse section varies depending on the position of the plate inside the cavity.
The upper face of the cavity can be closed by a cover having a central passage giving access to a connector member connecting the plate to the free end of the deformable rod and the periphery of the flexible membrane can be clamped between the cover and the upper edge of the cavity, the central area of this membrane being clamped between the plate and the connector member.